The present invention relates to a pressure and temperature transducer, and in particular to a piezoelectric, dual mode transducer suitable for use in a borehole environment.
Piezoelectric pressure and temperature transducers have been known for some time. Such transducers typically comprise a quartz crystal resonator located inside a housing structure. Electrodes are placed on opposite sides of the resonator to provide a vibration-exciting field in the resonator. Environmental pressure and temperature are transmitted to the resonator via the housing and the stresses in the resonator alter the vibrational characteristics of the resonator, this alteration being sensed and used to interpret the pressure and/or temperature. U.S. Pat. No. 3,617,780 (see FIG. 1) describes one form of such a transducer which comprises a unitary piezoelectric crystal resonator and housing structure 10 in which the resonator 12 is positioned on a median (radial) plane of the cylindrical housing 14. Crystal end caps 16, 18 are located at either end of the housing 14 to complete the structure of the transducer. Since the vibration of the resonator 12 is affected by both temperature and pressure, such devices can be difficult to use in environments where both vary in an uncontrolled manner. Such devices are known as single mode transducers.
One proposal to overcome this drawback of single mode transducers is described in U.S. Pat. No. 5,471,882. In this case and instrument is provided with two single mode transducers configured to have different temperature responses but similar pressure responses. By comparing the output of the two, the temperature effect can be cancelled. Another approach is to isolate one of the transducers from the environment to provide a reference against which the other can be calibrated.
U.S. Pat. Nos. 4,547,691 and 5,394,345 describe dual-mode transducers, an example of which is shown in FIG. 2. The resonators in such transducers have two vibrational modes at different frequencies, known as C mode and B mode. C mode is responsive to both pressure and temperature variation whereas the B mode is primarily responsive to temperature, the effect of pressure being relatively small. The structure of the dual-mode transducer again has a unitary resonator and housing structure 20. However, in this case, the resonator 25 lies in an axial plane of the cylindrical housing 22, the ends 26, 27 of the resonator 25 being unconnected to the housing 22. Again, electrodes are located on opposite faces of the resonator 25 to excite the vibrational behaviour.
U.S. Pat. No. 4,562,375 describes dual-mode and multiple-mode transducers, wherein the resonators in such transducers have at least two vibrational modes. U.S. Pat. No. 6,147,437 describes a transducer and tool for use in borehole environments with high temperatures and pressures.
Pressure and temperature transducers such as these find uses in borehole measurement tools such as are used in oil or gas wells. One example is the MDT Modular Formation Dynamics Tester of Schlumberger. One characteristic of oil and gas wells is that often relatively high temperatures and pressures are encountered. Also, the size of the tools is limited and it is important that the pressure and temperature measurement is relatively quick in order to allow fast measurements along the whole length of the well.
It is an object of the present invention to provide a transducer which can be made to withstand high temperatures and pressures and have a relatively small size and fast response time when applicable.
A transducer according to this invention comprises a central resonator section that is generally cylindrical in shape and has a resonator element located in a radial plane and end caps secured to the ends of the resonator section, the end caps having a base section with an outer wall extending around the periphery of the base section to define a cavity; and is characterised in that the end caps include an inner wall extending across the cavity within the outer wall.
Preferably the end cap base section has the same general shape (approximately circular) as the resonator section and the inner wall connects the outer wall across a diameter of the end cap. The inner wall can be of different dimensions to the outer wall. For example, the inner wall might be of lesser height and/or different thickness to the outer wall. The presence of the inner wall means that the end cap will be stiffer in one direction than another and so will transmit different stresses to the resonator according to the direction of the inner wall (an alternative view is that the support of the resonator section by the end caps against deformation is greater along the axis of the inner wall than orthogonal to this axis). Stresses along the axis of the inner wall will be poorly transmitted to the resonator whereas stresses orthogonal to this will be transmitted relatively easily.
The resonator is typically circular in shape and, as is common in the art, formed in a unitary fashion with the housing. However, the shape of the resonator can be chosen to suit requirements and may be formed separately from the housing. The end caps are likewise typically formed from a single piece of piezo-electric quartz crystal
The position, size and shape of the internal wall is chosen so as to maximise stress contrast between orthogonal transverse axes of the end cap and hence maximise deformation in one direction and minimise deformation in the other direction on application of pressure to the transducer. The end caps in turn provide mechanical support to the resonator section so modifying its response to pressure accordingly. The resonator can be connected to the housing around all of its periphery or only in two regions to further emphasise the difference in response to applied pressure.
For a dual mode resonator, it is desirable that the sensitivity of the pressure/temperature sensitive mode (C mode) is aligned with the axis of maximum deformation and the temperature sensitive mode (B mode) is aligned with the axis of minimum deformation.
By adopting a transducer of the present invention, it is possible to produce a much smaller sensor which still retains accuracy, resolution and fast dynamic response when compared to the prior art designs.
It will be appreciated that changes can be made to the transducer while still remaining within the scope of the invention. The shape of the resonator can be selected according to requirements. A bi-convex shape is preferred for good energy trapping but planar or plano-convex section could also be used.
Many of the choices outlined above will depend upon the cut angles of the piezoelectric material of the resonator. The preferred material for the transducer of the invention is crystal quartz with a double rotation cut of angles xe2x96xa1=24xc2x0, xe2x96xa1=33xc2x0 (WAD cut).